Interdigital transducers have made possible a number of useful surface acoustic wave devices such as filters, resonators, delay lines and the like. A pair of interdigital transducers at the input and output ends of a surface acoustic wave channel, for example, comprise an acoustic delay line useful in various known signal-processing applications. If the transducers are sufficiently frequency-selective, the delay line acts as a filter which is useful in communications applications.
But the efficiency of such devices is materially reduced if the transducer which generates the acoustic signal sends only half of its acoustic output in the direction of the receiving transducer. The balance of the acoustic energy is transmitted in the opposite direction and is therefore wasted. If this oppositely directed energy is strongly reflected back to the receiving transducer in phase with the forward wave, a large increase in the efficiency of the device results. Similar considerations apply to the acoustic energy which leaks through the receiving transducer. A half power loss represents 3 dB; and since this happens at each of the two transducers, the total loss is 6 dB.
A surface wave acoustic resonator comprises a transducer which is both sender and receiver, located within a resonant cavity that is defined by a pair of opposed acoustic reflectors. The efficiency of the device is heavily dependent upon the efficiency of the reflectors, because they determine how much of the acoustic standing wave is reflected back into the cavity at both ends.
The inefficiency of prior art reflectors, such as grooves or metal lines, forces a trade-off between reflectivity and bandwidth. If a phased array of grooves or metal lines is used as a reflector, a large number of them is required to develop a reflection of useful amplitude. But the larger the number of lines, the narrower is the bandwidth of the device, because a large number of lines can all reflect in phase only within a relatively narrow frequency range. In prior art devices, quite often the desired bandwidth is incompatible with a high reflection coefficient.
Piezoelectric materials are able to maintain remanent electric states at a microscopic (domain) level. It has been recently recognized that when adjacent domains are oppositely polarized, the domain boundary is a strong reflector of acoustic waves. See "Filamentary Domain-Reversal Defects in Y-Z LiNbO.sub.3 ; Structure, Composition, and Visualization Techniques" by Miller, et al., and "Acoustic Effects of Filamentary Defects in Y-Z LiNbO.sub.3 " by de Vries, et al. Miller and de Vries are the present inventors; their papers were presented at the Ultrasonics Symposium in Boston, Sept. 26-28, 1979 and later published as part of the Proceedings thereof. Both of these papers regard such domain reversals as accidental defects which degrade the performance of surface wave acoustic devices when they occur; but the de Vries paper recognizes that "If reversed domains could be obtained in a reproducible way, insertion loss of filters could be drastically reduced while the filter would become smaller also." The prior art, however, seems not to have harnessed the reflective capability of domain reversals in a useful way.
According to this invention, remanent electric polarity reversals are distributed in a pattern which is useful as an effective surface acoustic wave reflector, with the result that both high efficiency and wide bandwidth can be achieved in acoustic devices such as delay lines, filters, resonators, and the like.
Such reflectors also make it possible to build smaller surface acoustic wave devices for a given level of reflectivity, and they lend themselves readily to phase compensation schemes for the reduction of triple transit reflections, a serious problem in intermediate frequency filters used for television reception.
The invention will now be described in detail in conjunction with the following drawings.